WMO: “This should serve as yet another wakeup call about the constantly rising levels of greenhouse gases which are driving climate change. If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat trapping gases.”

GENEVA, Switzerland, May 26 (UPI) -- The World Meteorological Association (WMO) announced Monday that the average monthly concentration of carbon dioxide (CO2) in the northern hemisphere exceeded 400 parts per million (ppm) in April, the highest monthly average on record.

A press release from the United Nations' weather agency says crossing the "threshold is of symbolic and scientific significance and reinforces evidence that the burning of fossil fuels and other human activities are responsible for the continuing increase in heat-trapping greenhouse gases warming our planet."

According to the WMO's release, CO2 levels have risen more than 40 percent, up from pre-industrial levels of around 278 ppm, since human's began burning fossil fuels. CO2 can remain in the atmosphere for hundreds of years, longer still in the oceans. Because plants absorb more carbon dioxide during summer months when their foliage is more dense and plentiful, CO2 levels fluctuate from season to season and tend to peak in the spring. The northern hemisphere, due to higher levels of human industrial activity than the southern hemisphere, tends to have a more pronounced seasonal cycle.

But even with April's reading representing a seasonal peak, Earth's atmosphere hasn't seen levels as high as 400 ppm for millions of years.

"This should serve as yet another wakeup call about the constantly rising levels of greenhouse gases which are driving climate change. If we are to preserve our planet for future generations, we need urgent action to curb new emissions of these heat trapping gases," WMO Secretary-General Michel Jarraud said in a statement. "Time is running out."

United Press International is a leading provider of news, photos and information to millions of readers around the globe via UPI.com and its licensing services.

With a history of reliable reporting dating back to 1907, today's UPI is a credible source for the most important stories of the day, continually updated - a one-stop site for U.S. and world news, as well as entertainment, trends, science, health and stunning photography. UPI also provides insightful reports on key topics of geopolitical importance, including energy and security.